During the past ten years the requirements for cleanliness in semiconductor processing equipment have increased at least a hundred times. In the past three years alone requirements have increased ten fold. There is little doubt that the industry will need "absolute" cleanliness, that is to the limit of current measurement, in the next several years.
Semiconductor feature sizes have been cut in half in the past few years and packing densities have doubled or tripled in the same time period. It also appears that the rate of change is accelerating rather than holding at past rates. With these changes, the problems caused by contamination in semiconductor processing become even more serious.
Cleanliness is also important in the health and pharmaceutical industries, driven by the need to reduce the contamination of treatment processes. In view of these requirements, there is a need for gas flow equipment such as valves and regulators, for use in semiconductor processing and in the health and pharmaceutical industries, which is cleaned to previously unheard of levels.
In the past, such components have been cleaned almost universally by use of solvents. In addition to the problems of atmospheric pollution and operator health hazards, solvents do not clean absolutely. They leave films and particle residuals. Ultrasonic cleaning may also drive particles into crevices in instrument parts, for a later release. Chlorofluorocarbon cleaning solvents sold under the trademark Freon are examples of known cleaning solvents as well as 1,1,1-trichloroethane and methylene chloride.
Currently available, specialty-cleaned, double-bagged regulators and valves (off-the-shelf) cleaned with conventional solvents and methods have been tested for cleanliness by passing dry nitrogen gas, filtered through a 0.01 .mu. filter, through the components and then through a laser particle counter for detecting particles. The results showed particle counts of 100 to 1,000 or more particles of 0.1 .mu. diameter or larger per cubic foot of gas flow. These dirty or contaminated components will not clean up once installed in a gas flow system even though large amounts of dry nitrogen or argon gas are passed through the system. Thus, the components become sources of contamination during semiconductor wafer manufacture or other associated process.
The lack of cleanliness of the components cleaned by conventional solvents, methods and apparatus is also problematical where active ions and organic contamination such as organic films remain on the components. Active ions, e.g. metallic ions, can adversely affect the process in which the equipment is to be used. Organic contamination will not show up in the aforementioned particle count with a laser particle counter but can react in a treatment process to produce harmful effects.
In the past it has been postulated that such residual contamination was due to "shedding" of particles from a soft seat. Applicant's research in connection with the present invention has shown this not to be true and that an absolute level of cleanliness can be achieved by means of the present invention.
It is well known to use of choline (trimethylhydroxyethylammonium hydroxide) and hydrogen peroxide in a de-ionized water solution for cleaning silicon wafers in semiconductor production. One early formulation was known as the "RCA" cleaning method because of its discovery at RCA many years ago. Japanese Patents 63-114,128 and 63-272,149 involve similar solutions, in some cases with the addition of a surfactant.
Choline based cleaning is excellent for silicon wafers. However, the results of research by applicant in connection with the present invention indicate that choline gives poor results when cleaning stainless steel and plastic. Choline is unstable at the high processing temperatures required for stainless steel cleaning with the method of the present invention as discussed below. Also, hydrogen peroxide cannot be safely heated to near boiling temperatures in a mixture with an organic solute. An "RCA" type bath must necessarily be used at bath temperatures under 50.degree. C.
German Patent DE 3,610,592 discloses a sprayable oven and grill cleaner for removing baked on food residues wherein tetraalkyl ammonium hydroxide is substituted for the usual sodium hydroxide with the solvent of plain water, not de-ionized water. This strong, but crude cleaner is not applicable to instrument products for obtaining absolute or near-absolute cleanliness.